Spark gaps adapted to generate an arc between the electrodes, and with a careful time determination, are utilized, inter alia, in high-voltage laboratories for triggering laser beams and as protection for series capacitors in electric power lines. The present invention is primarily intended for applications within the latter field but is not in any way limited thereto.
Series capacitors are used in electric power lines, primarily for increasing the transmission capability of a power line. Such series capacitor equipment comprises a capacitor bank that is connected to the power line and is traversed by the current of the power line. The voltage across such a series capacitor becomes proportional to the current in the power line, and in case of an overcurrent in the power line, for example caused by a short circuit in the power network, an overvoltage arises across the series capacitor. It is previously known, for the purpose of protecting the capacitor from such overvoltages, to connect the capacitor in parallel with a spark gap that is triggered in a suitable manner in case of an overvoltage across the capacitor. In this way, the line current is shunted past the capacitor, which in this way is protected. Known protection devices of this kind are described, for example, in U.S. Pat. No. 4,625,254, U.S. Pat. No. 4,652,963, U.S. Pat. No. 4,703,385, U.S. Pat. No. 4,860,156, U.S. Pat. No. 5,325,259.
U.S. Pat. No. 4,625,254 describes a device comprising a linear resistor that is series-connected to a voltage-dependent metal-oxide varistor (MOV). The series-connected resistor elements are connected in parallel with the series capacitor in a high-voltage network to achieve an overvoltage protective circuit for the series capacitor. Further, a spark gap is connected in parallel with the series-connected resistor elements in the event of overloading thereof. The voltage across the linear resistor triggers a device for igniting the spark gap when the voltage across the linear resistor exceeds a predetermined voltage. The resistance of the linear resistor and of the varistor is so dimensioned that the predetermined voltage constitutes the smaller part of the voltage across the capacitor.
U.S. Pat. No. 4,652,963 describes a series capacitor bank for connection to an electric network, whereby the capacitor bank is provided with equipment for overvoltage protection, which has two branches connected in parallel with the capacitor bank. The first branch comprises a zinc oxide varistor in series with a linear resistor and the second branch comprises a varistor with a higher voltage knee than the first zinc oxide varistor. The resistance of the linear resistor is preferably of the same order of magnitude as the absolute value of the impedance of the capacitor bank at a frequency corresponding to that of the network.
U.S. Pat. No. 4,703,385 describes an overvoltage protection device for a series capacitor in a high-voltage network. A voltage-dependent resistor composed of a number of MOVs are connected in parallel with the capacitor. In parallel with the resistor is a spark-gap member, which consists of two series-connected spark gaps for shunting the resistor in the event of overloading therein. The energy for triggering the spark-gap member is obtained from an extra capacitor that is charged during operation and is supplied to one of the spark gaps via a switching member. The switching member is controlled by an overvoltage detector and a pulse transformer. An MOV is connected in series with the high-voltage winding of the transformer. The transformer is connected such that the trigger pulse is directed opposite to the voltage across the series capacitor.
U.S. Pat. No. 4,860,156 describes an overvoltage protection device for series capacitors with the aid of spark gaps. The protection device comprises a triggering circuit for a spark-gap chain of at least two spark gaps, one of which is provided with at least one triggering electrode. A resistor chain is connected in parallel with the spark-gap chain and comprises at least two series-connected resistor groups. That of the resistor groups that is connected in parallel with that of the spark gaps that has a triggering electrode includes a voltage-dependent resistor composed of zinc-oxide varistors that are connected in series with the linear resistor. The voltage across the linear resistor is supplied to the triggering electrode of the spark gap to ignite the spark gap when this voltage amounts to a predetermined value.
One disadvantage of conventional ignition of the arc in the main spark gap based on an auxiliary spark gap, that is, where the main spark gap is triggered to ignite via a spark generated by a triggering circuit, is that it requires a very high voltage across the main spark gap. The reason for this is that the mode of operation is based on the auxiliary spark gap substantially serving to ionize the air between the main electrodes. The ionization facilitates the formation of an arc between these; however, it assumes that the voltage is sufficient for a flashover to arise. The voltage across the main spark gap must amount to at least some ten kV. This limits the possibilities of application. Further, it requires reconditioning of the spark gap even after a few discharges because the corrosion caused by the arc on the electrodes results in the electrode distance being influenced, which, in the case of such a conventional kind of spark-gap triggering, influences the tripping level, that is, at which voltage across the main spark gap that an arc is formed.
U.S. Pat. No. 5,325,259 describes an overvoltage protection device for a series capacitor that has a main spark gap and an auxiliary spark gap, associated therewith, for ignition of the main spark gap. A second auxiliary spark gap is arranged close to the first auxiliary spark gap for ignition thereof. The auxiliary spark gaps are connected between one of the electrodes of the main spark gap and a voltage divider comprising resistors and a varistor. When exceeding the voltage knee of the varistor, the second auxiliary spark gap is ignited, the arc of which in its turn moves towards and ignites the main spark gap. During the burning time of the spark gap, a controlled discharge of the series capacitor through a resistor takes place.
During the triggering according to U.S. Pat. No. 5,325,259, the arc formation in the main spark gap is not exclusively dependent on ionization in the spark gaps. The first and second spark gaps are so arranged that a certain arc travelling effect is achieved upon ignition of the second auxiliary spark gap by the first auxiliary spark gap and upon ignition of the main spark gap by the second auxiliary spark gap. In this way, the voltage required for maintaining an arc across the main spark gap is lower than in conventional spark gaps. This reduces, to a certain extent, the above-mentioned disadvantages associated with the high voltage required between the main electrodes when using conventional technique. However, there is still a need of a relatively high, although moderate, voltage between the main electrodes.
Therefore, this does not eliminate the disadvantages resulting from the fact that the air has a relatively short sparking distance and hence may be easily re-ignited. Further, there is a risk that the plasma formed in the main spark gap may reach to the auxiliary electrodes and damage these.
The object of the present invention is to eliminate the disadvantages associated with the prior art for igniting an arc in a spark gap.